By-pass or turbo-fan engines

For a given amount of thrust, the Froude efficiency can be improved by increas­ing the mass flow rate of air, while reducing the jet speed. This can be achieved by increasing the size of the low pressure compressor stage, and by-passing some of the compressed air around the outside of the combustion chamber and turbine, as illustrated in Fig. 6.23. As in this illustration, separate spools are normally used for the low pressure by-pass and high pressure core stages. The hot and cold jets are arranged to have about the same velocity at exit.

The low pressure by-pass stages are effectively ducted fans, and by-pass jet engines are described as turbo-fans. In Britain, the term was originally only applied to high by-pass ratio engines (described below).